1. Field of the Invention
The present invention relates to a modulator and a demodulator used in Orthogonal Frequency Division Multiplex (hereinafter referred to as OFDM) transmission, and more particularly to a phase correcting technique.
2. Description of the Background Art
Recently, a transmission system using the OFDM technique is attracting attention in digital audio broadcasting for mobile devices and terrestrial digital television broadcasting. The OFDM technique is a kind of multi-carrier modulation system, in which data to be transmitted (transmitted data) is assigned to a large number of subcarriers so arranged that adjacent subcarriers are orthogonal to each other and converted into a digital modulated signal in the time domain by inverse Fourier transform to generate an OFDM signal. In the OFDM transmission, the OFDM signal generated by applying the above-described processing to the transmitted data on the transmitting end is transmitted to the receiving end. The receiving end applies the reverse process to that applied on the transmitting end to the transmitted OFDM signal to reproduce the transmitted data. Since each data divided onto the subcarriers has a longer period, the OFDM signal is not susceptible to effects of delayed waves such as multipath.
The OFDM demodulation is achieved by applying Fourier transform by using a fast Fourier transform (hereinafter referred to as FFT) circuit to the OFDM signal down converted to the baseband in a quadrature detector circuit. At this time, in the quadrature detector circuit, frequency synchronization must be accurately established between the transmitter and receiver, and in FFT, one symbol section must correctly be taken in from the received OFDM signal with a given clock, to obtain the phase and amplitude information of the subcarriers through the Fourier transform.
When a frequency error occurs in the OFDM signal on the transmitting and receiving ends, or when a timing error occurs and one symbol section cannot accurately be captured, the subcarriers suffer phase rotation and then the transmitted data cannot be reproduced. In this way, the OFDM demodulation requires accurate frequency synchronization, symbol synchronization, and clock synchronization, and a conventional OFDM demodulator must establish the frequency synchronization, symbol synchronization, and clock synchronization by using synchronizing symbols. Accordingly, as shown in FIG. 11, in the OFDM signal So, a transmission frame is composed of a plurality of OFDM symbols OS each including a plurality of ( . . . , k, k+1, k+2, . . . ) subcarriers SC, which is transmitted with a synchronizing symbol inserted in each frame. In the drawing, the vertical axis shows the phase xcfx86 and the horizontal axis shows the subcarrier frequency F. The character xcex94 xcfx86 denotes a phase error between adjacent subcarrier.
As shown in FIG. 12, a null symbol is inserted as the synchronizing symbol RS in the beginning of n (n is an integer of one or larger) OFDM symbols OS to form one transmission frame Fr, and the null symbols are successively detected to establish synchronization. A signal from which synchronizing information can easily be obtained can be used as the null symbol, and it does not necessarily have to be an OFDM modulated signal. That is to say, since the OFDM signal So presents a waveform like random noise, it is difficult to obtain the synchronizing information directly from the time-domain waveform. Accordingly, a sine-waveform signal can be used for the frequency synchronization, and a waveform signal modulated by an amplitude shift keying (ASK) scheme, from which clock components can be easily extracted, can be used for the clock synchronization.
Referring to FIG. 13, the concept of the OFDM signal thus structured will be described. The left half of the diagram schematically shows the state SF of the OFDM signal in the frequency domain and the right half shows the state ST in the time domain. In the frequency-domain signal SF, a large number of subcarriers SC are orthogonally arranged on the frequency axis F in each OFDM symbol OS1 to OSn. This frequency-domain signal SF is subjected to inverse Fourier transform and OFDM modulation to generate the time-domain signal ST. The subcarriers SC are arranged at intervals P=1/PS (Hz) primarily modulated on the transmitting end, which are transformed by inverse Fourier transform to the signal ST having the symbol periods PS (sec) on the time axis T.
The transmitting end forms an OFDM signal transmission frame with the OFDM symbols OS and synchronization reference symbol RS and sends the frame. The synchronization reference symbol RS does not necessarily have to be an OFDM modulated symbol, but may be a signal having a waveform easy to use in synchronizing processing.
On the receiving end, only the synchronization reference symbol RS is taken out from the input signal ST on the time axis for synchronizing control. The OFDM symbols OS are cut out for each symbol period PS and converted by Fourier transform into the signal SF on the frequency axis, and thus the OFDM symbol OS is separated into the subcarriers SC. Subsequently, primary demodulation (data demodulation) is applied to the separated subcarriers to obtain the received data or to reproduce the transmitted data. In order to accurately maintain the synchronization in such OFDM signal demodulating process, the synchronization reference symbols must be transmitted periodically.
Referring to FIG. 14, the OFDM demodulator disclosed in Japanese Patent Laying-Open No. 8-102769 will now be described as an example of such a conventional OFDM demodulator. The OFDM demodulator DMC includes an A/D converter 101, a clock synchronization establishing portion 102, a quadrature detector 103, a frequency synchronization establishing unit 104, a Fast Fourier Transform unit (FFT) 105, a symbol synchronization establishing unit 106, and a primary demodulator 107. The OFDM signal So"" sent from the transmitter is supplied to the A/D converter 101, clock synchronization establishing unit 102, frequency synchronization establishing unit 104, and symbol synchronization establishing unit 106.
The clock synchronization establishing unit 102 detects a synchronization error in a sampling clock between the transmitter and receiver in the OFDM signal on the basis of the synchronizing symbols RS in the OFDM signal Soxe2x80x2. The clock synchronization establishing unit 102 then corrects the detected synchronization error to generate a synchronized sampling clock signal Ssc and outputs the same to the A/D converter 101. The A/D converter 101 converts the analogue OFDM signal Soxe2x80x2 to a digital OFDM signal So synchronized in sampling clock component on the basis of the sampling clock signal Ssc and outputs the signal So to the quadrature detector 103.
The frequency synchronization establishing unit 104 detects a synchronization error in carrier signal frequency between the transmitter and receiver on the basis of the synchronizing symbol RS in the OFDM signal Soxe2x80x2 and generates and outputs a synchronized frequency signal Scf to the quadrature detector 103. The quadrature detector 103 subjects the OFDM symbol OS (subcarriers SC) to quadrature detection in the digital OFDM signal So synchronized in sampling clock component on the basis of the frequency signal Scf, converts it from the intermediate frequency band into the OFDM signal Sb in the baseband, and outputs the signal Sb to the fast Fourier transform unit 105. Needless to say, the OFDM signal Sb in the baseband is synchronized in carrier signal frequency component and also synchronized in sampling clock component.
The symbol synchronization establishing unit 106 detects a synchronization error in symbol time window between the transmitter and receiver on the basis of the synchronizing symbol RS in the OFDM signal Soxe2x80x2 to generate a synchronized symbol time window signal Sst and outputs the signal Sst to the fast Fourier transform unit 105. The fast Fourier transform unit 105 applies fast Fourier transform to the OFDM signal Sb in the baseband on the basis of the symbol time window signal Sst. The fast Fourier transform unit 105 separates the signal in the time domain to the subcarriers SC in the frequency domain for each OFDM symbol OS to generate a symbol synchronized subcarrier signal Sc and outputs the signal Sc to the primary demodulator 107. This subcarrier signal Sc is synchronized in symbol window and also synchronized in sampling clock and carrier signal frequency.
The primary demodulator 107 demodulates the subcarrier signal Sc outputted from the fast Fourier transform unit 105 for each subcarrier to reproduce the transmitted data Sd.
The conventional OFDM demodulator DMC applies Fourier transform by using the fast Fourier transform (FFT) unit 105 to the OFDM signal Sb down converted to the baseband by the quadrature detector circuit 103. At this time, the quadrature detector circuit 103 requires accurate frequency synchronization between transmitter and receiver, and the FFT accurately captures one symbol section from the received OFDM signal with a defined clock to obtain the phase and amplitude information of the subcarriers by Fourier transform.
In the data processing in the OFDM demodulator, the same conditions as those in the transmitter must be correctly reproduced about the sampling clock, carrier frequency, and FFT symbol window time. That is to say, in OFDM demodulation, synchronization must be established about the sampling clock, carrier frequency, and symbol time window. The conventional OFDM demodulator establishes the symbol synchronization and clock synchronization by detecting the synchronizing symbols intermittently inserted at certain intervals. In this case, the synchronizing symbols for several frames must be detected before establishing synchronization, and the OFDM symbols in this period cannot be correctly demodulated. However, the synchronization errors between the transmitter and receiver easily occur due to variations in the transmission environment and the like, which cause clock error, frequency error, and time window error. When such errors are occurring, the carriers of the OFDM modulated symbols suffer phase rotation of a amount corresponding to the errors with respect to the phases given at the time of transmission. Since information (transmitted data) are assigned to phases of the subcarriers, the transmitted data will be erroneously reproduced.
For these errors, the information is detected from the synchronization reference symbols, and the sampling clock error, carrier frequency error, and symbol time window error are fed back as the respective error signals to make adjustments for synchronization (to establish synchronization). Hence, since the subcarriers of OFDM symbols demodulated when the errors are occurring have phase rotation errors, the transmitted data are erroneously reproduced. Further, when the synchronizing symbols cannot be continuously detected at given intervals, stable synchronization cannot be established, and then it is very difficult to correctly demodulate the OFDM symbols transmitted in a burst manner.
Accordingly, the present invention has been made to solve the problems described above, and an object of the present invention is to provide a modulator and a demodulator for use in OFDM transmission which can correct phase errors of subcarriers to enable demodulation of OFDM symbols even when frequency error and timing error are occurring between the transmitter and receiver.
To achieve the object above, the present invention has the following features.
A first aspect of the present invention is directed to an OFDM demodulator receiving, as an input, an OFDM signal in which known pilot carriers as a phase reference are assigned to a plurality of given subcarriers among subcarriers used in transmission the demodulator comprises: a Fourier transform portion for separating the OFDM signal into the subcarriers by Fourier transform to generate a first subcarrier signal; a carrier phase error detecting portion for obtaining an amount of phase correction for each subcarrier in the first subcarrier signal on the basis of the pilot carrier in the first subcarrier signal; and a phase correcting portion for correcting a phase of the first subcarrier signal on the basis of the amounts of phase correction to generate a second subcarrier signal.
According to the first aspect described above, the amounts of phase correction to subcarriers in a Fourier transformed OFDM signal are obtained on the basis of pilot carriers having a reference phase and assigned to given subcarriers, and phase errors due to a synchronization error can be quickly corrected on the basis of the amounts of phase correction. Accordingly, when a plurality of OFDM symbols are transmitted in a burst manner, the individual OFDM symbols can be correctly demodulated.
According to a second aspect of the invention which depends on the first aspect, the carrier phase error detecting portion comprises: a pilot carrier location detecting portion for detecting location of the pilot carrier in the first subcarrier signal to generate a pilot carrier location signal; a pilot carrier extracting portion for extracting a first pilot carrier from the first subcarrier signal on the basis of the pilot carrier location signal; and a pilot carrier memory for holding the known pilot carriers and reading a second pilot carrier corresponding to the detected location among the held known pilot carriers on the basis of the pilot carrier location signal. The carrier phase error detecting portion further comprises: a phase difference calculating portion for calculating a phase difference between the first and second pilot carriers to generate a phase difference signal indicating the phase difference; a phase change amount calculating portion for calculating an amount of change of phase rotation between transmitting and receiving ends with respect to carrier frequency on the basis of the phase difference signal and generating an amount of phase difference change between transmitter-receiver signal indicating the amount of change; and a phase correction amount calculating portion for calculating an amount of phase correction for each subcarrier on the basis of the phase difference signal and the amount of phase difference change between transmitter-receiver signal and generating a phase error correcting signal.
According to the second aspect above, a phase difference of a pilot carrier between the transmitter and receiver is calculated and an amount of change of phase rotation between the transmitter and receiver with respect to the carrier frequency is then calculated, on the basis of which amounts of phase correction to the subcarriers are calculated. Hence, the amounts of phase correction correspond to the absolute phase errors of the subcarriers (the phase errors between the transmitter and receiver). Accordingly, with an OFDM signal in which the subcarriers are modulated by an absolute phase modulation, such as QPSK or QAM, the phase errors can be corrected so that the subcarriers can be correctly demodulated.
According to a third aspect of the invention which depends on the second aspect, the OFDM demodulator further comprises a data demodulating portion for demodulating the second subcarrier signal to reproduce transmitted data.
According to a fourth aspect of the invention which depends on the first aspect, the subcarriers in the OFDM signal to be inputted are subjected to differential modulation between the subcarriers adjacent in the frequency direction with reference to the pilot carriers. The carrier phase error detecting portion comprises: a pilot carrier extracting portion for extracting a pilot carrier from the first subcarrier signal; a phase calculating portion for calculating a phase of the pilot carrier on the basis of the extracted pilot carrier; a phase change amount calculating portion for calculating the amount of phase change from the calculated phase of the pilot carrier; and a phase correction amount calculating portion for calculating an amount of phase correction between the subcarriers on the basis of the calculated amount of phase change, wherein the phase correcting portion corrects the phase of the first subcarrier signal on the basis of the calculated amount of phase correction.
According to the fourth aspect above, an amount of phase correction between the subcarriers is obtained on the basis of the phase of the received pilot carrier, but the phase difference of the pilot carrier between the transmitter and receiver is not obtained. Accordingly, with an OFDM signal in which the subcarriers are subjected to differential modulation in the frequency direction, errors in phase difference between the subcarriers can be corrected with a simple structure to correctly demodulate the data.
According to a fifth aspect of the invention which depends on the fourth aspect, the OFDM demodulator further comprises a differential demodulation portion for reproducing transmitted data by subjecting the second subcarrier signal to differential demodulation.
According to a sixth aspect of the invention which depends on the fourth aspect, the OFDM demodulator further comprises an inter-carrier phase difference calculating portion for calculating a phase difference between the adjacent subcarriers on the basis of the first subcarrier signal, and the phase correcting portion corrects the phase difference indicated by an output of the inter-carrier phase difference calculating portion on the basis of the calculated amount of phase correction.
According to a seventh aspect of the invention which depends on the second aspect, the phase difference calculating portion obtains the phase difference between the first and second pilot carriers by receiving first and second complex numbers representing the first and second pilot carriers as inputs, multiplying the first complex number and conjugate complex number of the second complex number to obtain a third complex number, and calculating an arc tangent arctan (q/i) from real part i and imaginary part q of the third complex number.
According to an eighth aspect of the invention which depends on the second aspect, the phase difference calculating portion obtains the phase difference between the first and second pilot carriers by receiving first and second complex numbers representing the first and second pilot carriers as inputs, obtaining their phases "THgr"A and "THgr"B by an arc tangent arctan calculation, and calculating "THgr"A "THgr"B.
According to a ninth aspect of the invention which depends on the fourth aspect, the phase calculating portion obtains the phase of the extracted pilot carrier by calculating arc tangent arctan (q/i) from real part i and imaginary part q of a complex number representing the extracted pilot carrier.
According to a tenth aspect of the invention which depends on the fourth aspect, the phase calculating portion obtains an approximate value of the phase of the extracted pilot carrier by calculating q/i from real part i and imaginary part q of a complex number representing the extracted pilot carrier.
According to an eleventh aspect of the invention which depends on the second or fourth aspect, the phase change amount calculating portion obtains the amount of phase change with respect to the carrier frequency by estimating phase changes of other subcarriers from the phase of an arbitrary pilot carrier among the phases of the pilot carriers existing for each given carrier frequency.
According to a twelfth aspect of the invention which depends on the eleventh aspect, the phase change amount calculating portion obtains the amount of phase change with respect to the carrier frequency by interpolating the phase changes of other subcarriers from the phases of at least two pilot carriers among the phases of the pilot carriers existing for each given carrier frequency.
According to a thirteenth aspect of the invention which depends on the eleventh aspect, the phase change amount calculating portion obtains the amount of phase change with respect to the carrier frequency from a slope of a line obtained by linearly approximating the phase changes of other subcarriers from the phases of at least two pilot carriers among the phases of the pilot carriers existing for each given carrier frequency.
According to a fourteenth aspect of the invention which depends on the eleventh aspect, the phase change amount calculating portion obtains the amount of phase change with respect to the carrier frequency by dividing a phase difference between the phases of two pilot carriers among the phases of the pilot carriers existing for each given carrier frequencies by a difference in carrier frequency between the two pilot carriers.
According to a fifteenth aspect of the invention which depends on the eleventh aspect, the phase change amount calculating portion obtains the amount of phase change with respect to the carrier frequency by performing the calculation of dividing a phase difference between two pilot carriers among the phases of the pilot carriers existing for each given carrier frequencies by a frequency difference between the two pilot carriers a plurality of times for different pairs of the pilot carriers to obtain a plurality of division results within an OFDM symbol, and then averaging the plurality of division results.
According to a sixteenth aspect of the invention which depends on the first aspect, the pilot carriers in the OFDM signal are assigned to the subcarriers at constant frequency intervals.
According to a seventeenth aspect of the invention which depends on the first aspect, the pilot carriers in the OFDM signal are assigned to the subcarriers at frequency intervals increasing by given increments.
According to an eighteenth aspect of the invention which depends on the first aspect, the pilot carriers in the OFDM signal are assigned to the subcarriers at frequency intervals defined by a given PN sequence.
According to a nineteenth aspect of the invention which depends on the first aspect, the OFDM signal is continuously inputted.
According to a twentieth aspect of the invention which depends on the first aspect, the OFDM signal is inputted in a burst manner.
According to a twenty-first aspect of the invention which depends on the fourth aspect, a multi-valued differential phase shift keying is used as the differential modulation.
According to a twenty-second aspect of the invention which depends on the fourth aspect, a multi-valued differential amplitude and phase shift keying is used as the differential modulation.
A twenty-third aspect of the invention is directed to an OFDM demodulation method for demodulating an OFDM signal in which known pilot carriers as a phase reference are assigned to a plurality of given subcarriers among subcarriers used in transmission. The method comprises: a Fourier transform step of separating the OFDM signal into the subcarriers by Fourier transform to generate a first subcarrier signal; a carrier phase error detecting step of obtaining an amount of phase correction for each subcarrier in the first subcarrier signal on the basis of the pilot carrier in the first subcarrier signal; and a phase correcting step of correcting a phase of the first subcarrier signal on the basis of the amounts of phase correction to generate a second subcarrier signal.
According to a twenty-fourth aspect of the invention which depends on the twenty-third aspect, the carrier phase error detecting step comprises the steps of: detecting location of the pilot carrier in the first subcarrier signal to generate a pilot carrier location signal; extracting a first pilot carrier from the first subcarrier signal on the basis of the pilot carrier location signal; and holding the known pilot carriers and reading a second pilot carrier corresponding to the detected location among the held known pilot carriers on the basis of the pilot carrier location signal. The carrier phase error detecting step further comprises calculating a phase difference between the first and second pilot carriers to generate a phase difference signal; calculating an amount of phase change on the basis of the phase difference signal and generating an amount of phase difference change between transmitter-receiver signal; and calculating an amount of phase correction for each subcarrier on the basis of the phase difference signal and the amount of phase difference change between transmitter-receiver signal and generating a phase error correcting signal.
According to a twenty-fifth aspect of the invention which depends on the twenty-third aspect, the subcarriers in the OFDM signal to be inputted are subjected to differential modulation between the subcarriers adjacent in frequency direction with reference to the pilot carriers. The carrier phase error detecting step comprises the steps of: extracting a pilot carrier from the first subcarrier signal; calculating phase of the pilot carrier on the basis of the extracted pilot carrier; calculating an amount of phase change from the calculated phase of the pilot carrier; and calculating an amount of phase correction between the subcarriers on the basis of the calculiated amount of phase change, wherein the phase correcting step corrects the phase of the first subcarrier signal on the basis of the calculated amount of phase correction.
According to a twenty-sixth aspect of the invention which depends on the twenty-fifth aspect, the OFDM demodulation method further comprises an inter-carrier phase difference calculating step of calculating a phase difference between the adjacent subcarriers on the basis of the first subcarrier signal, and the phase correcting step corrects the phase difference between adjacent subcarriers calculated in the inter-carrier phase difference calculating step on the basis of the calculated amount of phase correction.
A twenty-seventh aspect of the invention is directed to an OFDM transmission system for transmitting an orthogonal frequency division multiplexed signal generated from a plurality of subcarriers modulated with transmitted data for each given-length symbol from a transmitting end to a receiving end through a wired or radio transmission path. The transmitting end comprises an OFDM modulator comprising: a data modulation portion for assigning known pilot carriers as a phase reference to a plurality of given subcarriers among transmission subcarriers used in the transmission of the orthogonal frequency division multiplexed signal and modulating data carriers which are the subcarriers other than the pilot carriers among the transmission subcarriers with the transmitted data; and an OFDM signal generating portion for generating the orthogonal frequency division multiplexed signal from the transmission subcarriers including the pilot carriers and the data carriers after modulation.
The receiving end comprises an OFDM demodulator comprising: a subcarrier separating portion for separating the subcarriers from the orthogonal frequency division multiplexed signal and outputting the subcarriers as received subcarriers; a phase error calculating portion for calculating an amount of phase correction for each received subcarrier on the basis of the pilot carrier included in the received subcarriers; and a phase correcting portion for correcting the phases of the received subcarriers in accordance with the amounts of phase correction.
According to a twenty-eighth aspect of the invention which depends on the twenty-seventh aspect, the orthogonal frequency division multiplexed signal is a burst-type signal composed of a plurality of symbols.
A twenty-ninth aspect of the invention is directed to an OFDM transmission system for transmitting an orthogonal frequency division multiplexed signal generated from a plurality of subcarriers modulated with transmitted data for each given-length symbol from a transmitting end to a receiving end through a wired or radio transmission path.
The transmitting end comprises an OFDM modulator comprising: a differential modulation portion for assigning known pilot carriers as a phase reference to a plurality of given subcarriers among transmission subcarriers used in the transmission of the orthogonal frequency division multiplexed signal and subjecting the transmission subcarriers to differential modulation between subcarriers adjacent in frequency direction with the transmitted data with reference to the pilot carriers; and an OFDM signal generating portion for generating the orthogonal frequency division multiplexed signal from the transmission subcarriers after the differential modulation.
The receiving end comprises an OFDM demodulator comprising: a subcarrier separating portion for separating the subcarriers from the orthogonal frequency division multiplexed signal and outputting the subcarriers as received subcarriers; a phase error calculating portion for calculating an amount of phase correction for each subcarrier on the basis of the pilot carrier included in the received subcarriers; a phase correcting portion for correcting the phases of the received subcarriers in accordance with the amounts of phase correction; and a differential demodulation portion for subjecting the received subcarriers after the phase correction by the phase correcting portion to differential demodulation with reference to the pilot carriers to reproduce the transmitted data.
According to a thirtieth aspect of the invention which depends on the twenty-ninth aspect, the orthogonal frequency division multiplexed signal is a burst-type signal composed of a plurality of symbols.
A thirty-first aspect of the invention is directed to an OFDM transmission system for transmitting an orthogonal frequency division multiplexed signal generated from a plurality of subcarriers modulated with transmitted data for each given-length symbol from a transmitting end to a receiving end through a wired or radio transmission path.
The transmitting end comprises an OFDM modulator comprising: a differential modulation portion for assigning known pilot carriers as a phase reference to a plurality of given subcarriers among transmission subcarriers used in the transmission of the orthogonal frequency division multiplexed signal and subjecting the transmission subcarriers to differential modulation between subcarriers adjacent in the frequency direction with the transmitted data with reference to the pilot carriers; and an OFDM signal generating portion for generating the orthogonal frequency division multiplexed signal from the transmission subcarriers after the differential modulation.
The receiving end comprises an OFDM demodulator comprising: a subcarrier separating portion for separating the subcarriers from the orthogonal frequency division multiplexed signal and outputting the subcarriers as received subcarriers; an inter-carrier phase difference calculating portion for calculating a phase difference between the subcarriers adjacent in the frequency direction among the received subcarriers; a phase error calculating portion for calculating an amount of phase difference correction which is an amount of correction to the phase difference between the adjacent subcarriers on the basis of the pilot carrier included in the received subcarriers; and a phase correcting portion for correcting the phase difference between the adjacent subcarriers in accordance with the amount of phase difference correction.
According to a thirty-second aspect of the invention which depends on the thirty-first aspect, the orthogonal frequency division multiplexed signal is a burst-type signal composed of a plurality of symbols.
A thirty-third aspect of the invention is directed to an OFDM modulator for transmitting an orthogonal frequency division multiplexed signal generated from a plurality of subcarriers modulated with transmitted data for each given-length symbol. The OFDM modulator comprises: a data modulation portion for assigning known pilot carriers as a phase reference to a plurality of given subcarriers among transmission subcarriers used in the transmission of the orthogonal frequency division multiplexed signal and modulating the transmission subcarriers with the transmitted data; and an OFDM signal generating portion for generating the orthogonal frequency division multiplexed signal from the transmission subcarriers after being modulated by the data modulation portion.
According to a thirty-fourth aspect of the invention which depends on the thirty-third aspect, the data modulation portion modulates data carriers which are the subcarriers other than the pilot carriers among the transmission subcarriers with the transmitted data.
According to a thirty-fifth aspect of the invention which depends on the thirty-third aspect, the data modulation portion subjects the transmission subcarriers to differential modulation between subcarriers adjacent in the frequency direction with the transmitted data with reference to the pilot carriers.
According to a thirty-sixth aspect of the invention which depends on the thirty-third aspect, the data modulation portion assigns the pilot carriers to the transmission subcarriers at constant frequency intervals.
According to a thirty-seventh aspect of the invention which depends on the thirty-third aspect, the data modulation portion assigns the pilot carriers to the transmission subcarriers at frequency intervals increasing in given increments.
According to a thirty-eighth aspect of the invention which depends on the thirty-third aspect, the data modulation portion assigns the pilot carriers to the transmission subcarriers at frequency intervals defined by a given PN sequence.
According to a thirty-ninth aspect of the invention which depends on the thirty-third aspect, the OFDM signal generating portion generates a continuous signal composed of a plurality of symbols as the orthogonal frequency division multiplexed signal.
According to a fortieth aspect of the invention which depends on the thirty-third aspect, the OFDM signal generating portion generates a burst-type signal composed of a plurality of symbols as the orthogonal frequency division multiplexed signal.
According to a forty-first aspect of the invention which depends on the thirty-fifth aspect, the data modulation portion performs a multi-valued differential phase shift keying as the differential modulation.
According to a forty-second aspect of the invention which depends on the thirty-fifth aspect, the data modulation portion performs a multi-valued differential amplitude and phase shift keying as the differential modulation.